Variability in the characteristics of cut-off low pressure systems over subtropical southern Africa

Abstract

Variability in the characteristics of cut-off low pressure systems over subtropical southern Africa is examined for the 1973–2002 period. These characteristics include their seasonality, frequency, duration, location and size. It is found that on average 11 cut-off lows occur over southern Africa south of 20°S per year and are most common in the March-May season. Potential relationships between the number of cut-off lows over southern Africa with the El Niño Southern Oscillation (ENSO), the Pacific South America pattern, the wave number 3 pattern and the semi-annual oscillation (SAO) are discussed. La Niña years appear to be associated with above average annual frequencies of cut-off lows but the reverse is generally not true for El Niño years. There was a shift in the preferred season for cut-off lows from March–May to June–August in the 1980s, which coincided with a weakening of the SAO and a shift in zonal wave number 3. This period also showed a change in the preferred location of these systems from southwestern subtropical southern Africa to the northeast of the region. The results suggest that there may be a relationship between cut-off lows over subtropical southern Africa, the wave number 3 pattern in the Southern Hemisphere and the SAO. Copyright © 2006 Royal Meteorological Society.